Nasa test-fires 3D-printed rocket parts

A 3D-printed injector plate delivers 20,000 lbs of thrust in a hot-fire test on 22 August

NASA

When we last left Tom Williams and his team of young engineers,
they were busy bringing monster 1960s-era rocket engines back to life. That
work continues to pay dividends, but Williams and the propulsion systems team at
Nasa's Marshall Space Flight Centre in Huntsville, Alabama, have a
wide variety of projects in the works at the moment. Their latest?
3D printing rocket components from scratch and firing them.

The test shown here,
which occurred on 22 August, involved an entire 3D-printed injector
plate -- the largest 3D-printed component Nasa has ever tested. It
delivered enough fuel and oxygen to produce 20,000 lbs of thrust
(about 89 kilonewtons), a bit more than you can get from an F-15's
Pratt and Whitney F100 turbofan
running at full military power.

Of course, Nasa's 3D printing doesn't have much in common with
the kind of home 3D printing I've spent the past few weeks
experiencing. While I've been faffing about with thermoplastics and fused
deposition modeling, Nasa has been busy forming metal powders
into solids with a technique called direct
metal laser sintering (referred to as DMLS or just MLS).
For their recent test, Nasa had a contractor build a duplicate of a
conventionally machined injector -- a structure that introduces
fuel and oxidiser into a rocket's combustion chamber.

The traditional 28-element injector consists of a housing and a
plate to divide the propulsion flow paths from the oxidiser flow
paths and 28 separately machined injector components. Each injector
is in turn made up of several elements joined together, including
an oxidiser feed path, a fuel feed path, and a connector to fix the
injector in place. The 3D-printed version, though, is all a single
monolithic piece from end to end. There are some minor differences
in design, but the 3D-printed version was designed to work with the
same propulsion flow paths and the same chamber pressure.

3-D Printed Injector Hot FiringNASA's Marshall Center

Fidelity is an issue with 3D-printed parts, even using advanced
techniques like MLS. Greg Barnett, the lead propulsion engineer on
the project, explained that DMLS introduces small surface
variations into the mix. "The surface is a little rougher," he
explained; however, those variations are within a consistent range
and can be compensated for in the design. In other words, each
DMLS-produced component doesn't have to be individually tested for
variation -- the "fix" to overcome the imperfections can be
designed into the component.

The test results on the 3D-printed components have been
extremely positive; Barnett and Williams told Ars Technica that the
3D-printed injector is equivalent in performance to the traditional
machined one. The next step is to move on to an injector with more
elements, which will mean testing with more power.

3D printing -- or "additive manufacturing," as it's called when
you get industrial like this -- is seen by Nasa as a vital way to
keep rocket component development costs down. In a lot of ways, the
ability to rapidly prototype via DMLS harkens back to the
Apollo-era development method of fast physical iteration. Rather
than spending a tremendous amount of time performing deep,
computer-based analyses of rocket components, Nasa can rough in a
design and then print and test a component within hours or
days.

The deep analysis and simulation tools are still available and
still used, but the months- or years-long physical manufacturing
time is drastically reduced. This gives engineers the flexibility
to design and build in the most optimal fashion. They can use
complex software analysis where necessary, but they don't have to
rely solely on computer modeling.

In the days of Apollo, Nasa operated with effectively unlimited
funding, which it used to create a nation-wide army of contractors
with tremendous manufacturing capabilities. Design-by-iteration was
feasible because there was so much design going on. These
days, the picture is entirely different. "It's almost a cultural
issue," explained Williams, "where a part can cost so much, you get
into what I call 'analysis paralysis.'" Without additive
manufacturing, prototype rocket parts that can withstand actual
hot-firing can cost so much and take so long to produce that when
you finally get a physical component to test, you're already hoping
the tests show that it's perfect -- otherwise it would take too
long to redesign. With additive manufacturing, that paralysis goes
away, and engineers can iterate as needed on actual physical
components.

Williams sees Nasa's work in this space as part of the service
that a responsible government agency should be providing. The
lessons learned will be available for use by any US-based company
that wishes to take advantage of them, which puts Nasa into the
role of a technology incubator and advanced research and
development shop.

"In 2008, about eighty percent of our workforce was focused on
supporting Shuttle and keeping the shuttle flying safely and on
getting Ares designed,"
he explained. Now, in the post-Shuttle and post-Constellation
world, "we're about forty percent focused on SLS.
So sixty percent of our workforce is working on research and
technology development: things that can help push the industry to
new levels that they wouldn't otherwise be able to achieve."

"And it's cool stuff," he finished. "The guys really enjoy it.
It's motivating and it's making a difference. It's really
exciting." The idea of Nasa being freed to play a government-funded
R&D role in rocket technology is a fascinating one and makes it
similar to the approach taken by many of the national labs run by
the Department of Energy. In this case, the knowledge and
techniques that result will be available to private space flight
companies like SpaceX, freeing them in turn to focus on delivering
economic returns.

In the near term, Nasa will continue pushing to develop additive
manufacturing for rocket components. One of the first major
applications will almost certainly be for the proposed SLS heavy
lift rocket. SLS' core stage will be powered by the legacy RS-25
Space Shuttle Main Engines, of which Nasa has a small existing
stockpile. If SLS is able to fly regularly, that inventory of
engines will quickly be exhausted and replacements will need to be
built. The RS-25 itself is an extremely efficient, extremely
complex machine, and utilising additive manufacturing could
potentially decrease its build costs by orders of magnitude.